Tumour infiltrating lymphocyte (TIL) therapy for treatment of refractory metastatic melanoma, has shown remarkable clinical efficacy in a number of clinical trials. However, extending the clinical benefit to patients with other cancers has posed a challenge. T cell anergy and exhaustion can be mediated by insufficient costimulation in the tumour microenvironment, in turn leading to loss of anti-tumour activity. Here, we describe the construction and functional testing of a chimeric costimulatory antigen receptor (CoStAR) which synergises with TCR signals in T cell and TILs upon antigen engagement. CoStAR consists of a tumour associated antigen specific single chain antibody fragment (scFv) fused to the signalling domains of CD28 and CD40. Transfer of a FRα specific CoStAR to T cells augments T cell activity in a manner that is strictly dependent on the provision of TCR-mediated signal 1. CoStAR also enhanced proliferation, even in the absence of exogenous IL-2. Using an in vivo tumour model, CoStAR improved T cell survival, enhanced control of tumour growth, and improved host survival. TIL from multiple cancer indications could be efficiently and reproducibly engineered with CoStAR, resulting in augmented activity in response to target antigen expressing cell lines and autologous tumour digest. CoStAR thus represents a novel approach to enhancing TIL activity via synthetic costimulation, and in turn increasing anti-tumour activity.